1. Field of the Invention
The present invention relates to a method of detecting a predicted failure in a fluid-pressure system and an apparatus for carrying out such a method.
2. Discussion of the Background
Heretofore, fluid-pressure systems such as pneumatic pressure operated systems employ pressure switches for supplying a negative and/or positive pressure.
For example, a negative pressure switch is incorporated in a pneumatic pressure operated system which is composed of pneumatic pressure passages and a pneumatic pressure device and which is operable under a negative pneumatic pressure (vacuum). When a workpiece is fed by a suction pad under a negative pressure from the negative pressure system, the negative pressure switch is used to confirm whether the workpiece is attracted to or released from the pressure pad.
One example of operation of such a vacuum pressure switch is shown in FIG. 1 of the accompanying drawings. The pressure switch is associated with a detecting circuit which is composed of a diffused transistor, an amplifier, an output circuit, and a rheostat. The detecting circuit transmits output signals Sa, Sb corresponding to pressure differences 1, 2 which are related to predetermined pressures at the time a workpiece is attracted and released, respectively. The pressure differences 1, 2 are employed to prevent the system from chattering.
The output signal Sa is produced according to a pressure threshold Ph2 which is of a relatively low vacuum, and the output signal Sb is produced according to a pressure threshold Phl which is higher in vacuum than the pressure threshold Ph2. The output signals Sa, Sb are supplied to a sequence controller which is connected to a computer for FA, for example, so that various control and drive means will be controlled.
The conventional vacuum pressure system usually does not have vacuum indicating means. When the pressure thresholds Phl, Ph2 are to be established, a workpiece is repeatedly attracted and released by the pressure pad, and then the pressure thresholds Phl, Ph2 are determined. Also, the rheostat is adjusted to supply output signals Sb, Sa corresponding to the determined pressure thresholds Phl, Ph2. More specifically, values set by the rheostat are supplied as reference signals to comparators, and compared thereby with actual measured values from the detecting circuit, thus producing pressure differences. If many pressure switches are combined with a pneumatic pressure system, then errors tend to be developed between pressure value settings for the pressure switches. Stated otherwise, it is highly difficult to rely on a quantitive approach to establish and adjust pressure difference settings. When the workpiece is repeatedly attracted and released, furthermore, a maximum vacuum level that can be reached is lowered with time because of the clogging of a filter and an ejector malfunction. As a result, the pneumatic pressure system frequently experiences a condition in which the vacuum fails to reach a predetermined vacuum level that is used to confirm the attraction of a workpiece (i.e., an illustrated attracted/released condition M).
Comparison of the output signal Sa for the purpose of an advance announcement of filter replacement or the like requires switch output signals (pulsed signals) to be latched. As a consequence, a complex signal processing arrangement is necessary and hence the entire system becomes large and complex. The above drawbacks also hold true for a fluid-pressure system with positive pressure switches.